The present invention relates to hydrocarbon cracking catalyst compositions, in particular relates to catalyst compositions which not only display high catalytic activity and gasoline selectivity when used in catalytic cracking of hydrocarbons but also have superior thermal and hydrothermal stability.
As the hydrocarbon cracking catalyst compositions, there have generally been used those comprising a crystalline aluminosilicate zeolite dispersed in a porous matrix such as silica, silica-alumina, silica-zirconia, silica-boria and the like. As is generally known, catalytic cracking of hydrocarbon is explained by reference to the carbonium-ion mechanism, wherein the acid point of a solid catalyst functions as the active site, but this acid point is easily poisoned by the alkali metals present in the catalyst. Due to this, it is customary that alkali metals have been removed from the cracking catalyst as much as possible by means of ion exchange or washing, and especially when water glass, sodium aluminate, crystalline aluminosilicate in the sodium form and the like are employed as the materials for preparing catalyst, largest efforts have been made in order to remove the sodium derived from these materials which is liable to remain in the catalyst.
Sodium is not preferable also in the point of promoting the sintering of catalysts. In the catalytic cracking process, the catalyst is exposed to high temperatures repeatedly because said process comprises the steps of using the catalyst normally at a reaction temperature of about 500.degree. C., burning the coke deposited on the catalyst during the reaction at a temperature of about 650.degree. C. for regenerating the catalyst, and then using said regenerated catalyst again in the reaction zone. Accordingly, when sodium remains in excess of its tolerable amount, it leads to the results that breakage of the crystals of crystalline aluminosilicate zeolite and sintering of the porous matrix are promoted by the sintering accelerating action of sodium, and the thermal and hydrothermal stability and the catalytic activity of the catalyst deteriorate.
In short, alkali metals typified by sodium have been considered not preferable as components of cracking catalysts, and therefore the amounts of alkali metals contained in the usual cracking catalysts have been minimized so as to be 1.0 wt. % or less, calculated as the oxide.